Ninety-nine million years ago in what's now Myanmar, a glob of tree resin oozed onto a beach. Today, the resulting fossilized lump of amber is giving scientists an astonishing glimpse into life on a Cretaceous coastline.

In a study published Monday in the journal PNAS, researchers led by Chinese paleontologist Tingting Yu reveal what is likely the first known record of an ammonite found in amber. These extinct marine mollusks were ancient relatives of octopuses and squid, and they didn't venture on land. Finding an ammonite shell in a land-formed fossil is therefore as eyebrow-raising as finding dinosaur remains on the bottom of an ancient seafloor.

“Amber—ancient resins from trees—commonly traps only some terrestrial insects, plants, or animals,” says study